Now going off of this which of the following will stack with it? Reserves? Premium time? Premium Tanks? The daily bonus?
In theory can I pop two of my 50% Credit/Battle Exp reserves, and a 300% crew reserve on top of my events, then play my premium tanks to maximize benefits? If so, are there any diminishing returns at certain points?

You can only use one credit boost at a time. But yes a 50 percent credit boost will stack with the bonus from the event. So you could make bank while running your premium tanks. I normally run a xp, credit, and crew training boost at the same time.